Uncertainty of atmospheric microwave absorption model: impact on ground-based radiometer simulations and retrievals

Abstract

Abstract. This paper presents a general approach to quantify absorption model uncertainty due to uncertainty in the underlying spectroscopic parameters. The approach is applied to a widely used microwave absorption model (Rosenkranz, 2017) and radiative transfer calculations in the 20–60 GHz range, which are commonly exploited for atmospheric sounding by microwave radiometer (MWR). The approach, however, is not limited to any frequency range, observing geometry, or particular instrument. In the considered frequency range, relevant uncertainties come from water vapor and oxygen spectroscopic parameters. The uncertainty of the following parameters is found to dominate: (for water vapor) self- and foreign-continuum absorption coefficients, line broadening by dry air, line intensity, the temperature-dependence exponent for foreign-continuum absorption, and the line shift-to-broadening ratio; (for oxygen) line intensity, line broadening by dry air, line mixing, the temperature-dependence exponent for broadening, zero-frequency line broadening in air, and the temperature-dependence coefficient for line mixing. The full uncertainty covariance matrix is then computed for the set of spectroscopic parameters with significant impact. The impact of the spectroscopic parameter uncertainty covariance matrix on simulated downwelling microwave brightness temperatures (TB) in the 20–60 GHz range is calculated for six atmospheric climatology conditions. The uncertainty contribution to simulated TB ranges from 0.30 K (subarctic winter) to 0.92 K (tropical) at 22.2 GHz and from 2.73 K (tropical) to 3.31 K (subarctic winter) at 52.28 GHz. The uncertainty contribution is nearly zero at 55–60 GHz frequencies. Finally, the impact of spectroscopic parameter uncertainty on ground-based MWR retrievals of temperature and humidity profiles is discussed.

RTTOV-gb v1.0 – Updates on sensors, absorption models, uncertainty, and availability

“…Coefficients for both R98 and R17 models are now available. Extending the results in Cimini et al (2018) from 60 to 150 GHz, Figure 1 shows clear-sky zenith downwelling T B computed with R17 model and the difference between T B computed with the two model versions, for six reference atmosphere climatology conditions. The difference spans from -2 to +3 K in the considered frequency range and thus it is not negligible for the sensors currently available for RTTOV-gb.…”

“…Conversely, RTTOV-gb was trained using a later version of MPM, described by Rosenkranz (1998, hereafter R98), which is probably the most used among the ground-based microwave radiometry community. This model is continuously revised and freely available (Rosenkranz, 2017 hereafter R17), and its uncertainty has been carefully investigated (Cimini et al, 2018). Therefore, RTTOV-gb has been trained using the R17 model also (version of 17/05/2017 available at http://cetemps.aquila.infn.it/mwrnet/lblmrt_ns.html ).…”

“…As mentioned, Cimini et al (2018) investigated the uncertainty of T B computed with R17 model due to the laboratory uncertainty of the adopted spectroscopic parameters. Through a sensitivity test, they identified 111 parameters (6 for water vapor and 105 for oxygen), whose contribution to the total uncertainty was dominant with respect to others.…”

Analysis of Water Vapor Absorption in the Far‐Infrared and Submillimeter Regions Using Surface Radiometric Measurements From Extremely Dry Locations

“…It is important to note that the adjustments made to TI‐sounding that are based on retrievals using radiometric measurements (hexagons in Figure ) are primarily performed in spectral regions in which we have more confidence in our spectroscopic knowledge than we have in the spectroscopy that we target in this study: Uncertainties are low for the spectroscopic parameters of the IR carbon dioxide bands, which are widely used for temperature retrievals from satellite observations. The properties of the 183‐GHz water vapor line and corresponding water vapor continuum also have modest uncertainties (Cimini et al, ; Payne et al, ; Payne et al, ) and measurements on this line have been relied upon in previous similar studies (e.g., Delamere et al, ). Water vapor line parameters and continuum from 400–550 cm −1 have been analyzed in Delamere et al () and improvements implemented, allowing this study to utilize this region with some confidence. …”

“…2. The properties of the 183-GHz water vapor line and corresponding water vapor continuum also have modest uncertainties (Cimini et al, 2018;Payne et al, 2008;Payne et al, 2011) and measurements on this line have been relied upon in previous similar studies (e.g., Delamere et al, 2010). 3.…”